The present inventions generally relate to medical imaging devices and methods, and more particularly to systems and methods for ultrasonically imaging body tissue.
For purposes of diagnosis and treatment planning, imaging techniques are commonly used in medical procedures to view the internal anatomy of a patient""s body. In one imaging technique, an imaging catheter with a distally rotatable ultrasound transducer is inserted into the patient""s body, e.g., through a blood vessel. Typically, the ultrasound transducer will be mounted on the distal end of a drive shaft that rotates within the catheter body. The drive shaft is typically composed of a counter wound spring coil to maximize the distal transfer and response of the rotational forces applied to the proximal end of the shaft, while maximizing lateral flexibility.
To obtain an interior image of the body tissue, the rotating ultrasound transducer transmits pulses of ultrasound energy into the body. A portion of the ultrasound energy is reflected off of the internal anatomy of the body back to the transducer. The reflected ultrasound energy (echo) impinging on the transducer produces an electrical signal, which is used to form a 360 degree cross-sectional interior image of the body. The rotating ultrasound transducer can be longitudinally translated, so that multiple cross-sectional images can be generated and later reconstructed into a three-dimensional interior image of the body tissue.
Oftentimes, it is desirable to localize the imaging catheter, so that it can be accurately guided within the target area of the patient""s body. In addition, it is also desirable to localize the imaging plane of the rotating imaging transducer, so that the resultant ultrasound image can be properly displayed within the context of the target area based on the localized position of the ultrasound transducer. In one guidance system, a graphical representation of the imaging catheter is displayed in a three-dimensional computer-generated representation of a body tissue, e.g., a heart chamber. The three-dimensional representation of the body tissue is produced by mapping the geometry of the inner surface of the body tissue in a three-dimensional coordinate system. For example, a mapping device can be placed in contact with multiple points on the body tissue to obtain surface points, and a graphical surface can then be conformed to the mapped surface points. Alternatively, a mapping device can be moved around inside the body cavity to obtain interior points, and a graphical surface can be gradually deformed in real time to include each interior point as it is obtained. In this case, the mapping device can also be placed in contact with the body tissue to obtain surface points, so that the graphical deformation process can be further refined.
In any event, the position and orientation of the imaging catheter, and thus, the position and orientation of the imaging plane, are determined by placing one or more location sensors on the catheter body known distances from the imaging transducer, and then tracking the position of these sensor(s) within the three-dimensional coordinate system. The position and orientation of the imaging plane can be extrapolated from the determined positions and/or orientation of the sensor(s). In the case of an ultrasound-based guidance system, multiple location sensors (in the form of ultrasound transducers) are placed along the distal end of the imaging catheter, so that its angular orientation can be determined. An example of this type of guidance system is the Realtime Position Management(trademark) (RPM) tracking system developed commercially by Cardiac Pathways Corporation, now part of Boston Scientific Corp. The RPM system is currently used in the treatment of cardiac arrhythmia to define cardiac anatomy, map cardiac electrical activity, and guide an ablation catheter to a treatment site in a patient""s heart.
Although the use of these types of guidance systems are generally useful in tracking the position and orientation of an imaging catheter and its imaging plane, it is still desirable to make further improvements. For example, the addition of each location sensor incrementally increases the complexity and cost of the system, and thus, a reduction in the number of location sensors needed to localize the distal end of an imaging catheter would be beneficial. In addition, the overall length of the counterwound drive shaft will vary as the catheter body is curved, thereby shifting the position of the imaging transducer proximally or distally relative to the catheter body a distance up to 8 millimeters. This may cause inaccuracies when determining the position of the imaging transducer within the three-dimensional coordinate system. Also, the extrapolation process used to determine the position and orientation of the imaging plane may cause further inaccuracies.
There thus remains a need for an improved system and method for localizing an imaging ultrasound transducer.
In accordance with a first aspect of the present inventions, a method of localizing an. ultrasound imaging transducer is provided. The method comprises operating the ultrasound imaging transducer in a first resonant mode to transmit an ultrasound imaging signal (e.g., a 9 MHz ultrasound pulse) and generating image data based on the transmitted imaging signal. For example, the image data generation can comprise processing a portion of the transmitted imaging signal that reflects off an object. The image data can represent any desired subject to be imaged, but in one preferred method, the imaging signal is transmitted into the body of a patient, in which case, the image data can be tissue image data. If the body of a patient is to be imaged, the imaging transducer is preferably introduced within the body of the patient. The imaging transducer can be located outside of the patient""s body, however, to externally obtain image data.
The method further comprises operating the imaging transducer in a second resonant mode to either transmit, receive, or transmit and receive, an ultrasound positioning signal (e.g., a 1 MHz ultrasound pulse), and determining a position of the imaging transducer based on the transmitted or received positioning signal, e.g., within a three-dimensional coordinate system. For example, the imaging transducer position determination can comprise calculating a distance between the imaging transducer and one or more ultrasound reference transducers by calculating a time period defined by the time of flight of the position signal between the imaging transducer and reference transducer(s). The imaging transducer can optionally be rotated around an axis so that the image data comprises 360xc2x0 cross-sectional image data. Although the present inventions should not be so limited in their broadest aspects, it can be appreciated that the operation of the imaging transducer in multiple modes allows the imaging transducer to be more accurately localized with or without the use of additional tracking transducers.
In one preferred method, the imaging transducer is conveniently operated in the first and second resonant modes by stimulating the imaging transducer with a single electrical signal. The imaging transducer can be operated in the two modes, however, by stimulating the imaging transducer with separate signals as well. In another preferred method, the imaging transducer exhibits a first isotropy radio when operated in the first resonant mode, and a second isotropy ratio greater than the first isotropy ratio, e.g., less than 10, when operated in the second resonant mode. In this manner, the positioning signal can be effectively received by any location surrounding the imaging transducer, and the imaging signal, itself, will be more focused.
In accordance with a second aspect of the present inventions, an ultrasound imaging transducer localization system is provided. The localization system comprises an ultrasound imaging transducer having first and second resonant modes, and control/processing circuitry coupled to the ultrasound transducer for operating the imaging transducer in the first resonant mode to transmit an ultrasound imaging signal (e.g., a 9 MHz ultrasound pulse), and for operating the imaging transducer in the second resonant mode to transmit or receive an ultrasound positioning signal (e.g., a 1 MHz ultrasound pulse). The control/processing circuitry is configured for generating image data based on the transmitted ultrasound imaging signal, and for determining a position of the imaging transducer based on the transmitted or received ultrasound positioning signal. The control/processing circuitry implemented in hardware, software, firmware, or any combination thereof.
In one preferred embodiment, the localization system further comprises a probe, such as, e.g., a catheter, on which the imaging transducer is mounted. In the preferred embodiment, the imaging transducer can be rotatable, in which case, the localization system can further comprise a drive unit a drive unit mechanically coupled to the imaging transducer for rotating the imaging transducer around an axis. In this manner, the control/processing circuitry can generate 360xc2x0 cross-sectional image data. The imaging transducer can, however, be non-rotatable. The control/processing circuitry can operate the imaging transducer in the first and second resonant modes, generate the imaging data, and determine the position of the imaging transducer in the same manner described above. The imaging transducer can exhibit the same characteristics as those previously described.